Metal production units, in particular for steel, at present integrate several metal production or treatment devices, if necessary regrouping them in a complete production line that extends from the treatment of the raw mineral to the production of finished products ready for marketing. Most of these metal production or treatment devices consume large quantities of compressed air (over 100 Nm.sup.3 of air per ton of metal) and/or gas from the air, notably oxygen (over 50 Nm.sup.3 per ton of metal) and/or a neutral gas (over 10 Nm.sup.3 per ton of metal). These air gases are generally supplied from liquefied gas containers or by gas pipelines. Besides, these air gases are produced by units for the separation of air gases, notably of the cryogenic type, which are also supplied with compressed air. Whether for the metal production or treatment devices or for the air gas separation units, the air compressors used are particularly heavy-duty items of equipment that consume a great deal of electrical energy, and because of this, considerably increase the production costs of such units.